[bprater]

Differential GPS is what drones use for light shows or mapping or anything requiring high fidelity. This gives them centimeter-level precision, at the cost of a large sensor array on both the ground and the drone. If you are adding this to a drone, you'd also need to include orientation and other parameters in your data stream, as you'd only be getting XYZ from GPS. Unfortunately, no simple hardware exists for your request.

[jvanderbot]

Pixhawk exists. https://pixhawk.org/

[ganzuul]

Differential GPS should be eminently hackable and cheap, including the antennas in question, but it's an open question if we even want that right now because spoofing would be no more difficult.

[MayeulC]

GPS-RTK is not that hard or expensive: https://docs.centipede.fr/ (in French).

https://docs.centipede.fr/docs/make_rover/rover_v5_1.html lists a €180 GPS module, which is not dirt-cheap, but still affordable.